Kinetics and mechanism of the bromination of phenols in aqueous solution. Evidence of general base catalysis of bromine attack

Abstract

The reactions of bromine with phenol, 4-bromophenol, and 4-methylphenol (p-cresol) in aqueous solution are catalyzed by carboxylate anions, confirming the suggestions of earlier work. The results are consistent with deprotonation of the phenol hydroxyl group by a general base occurring at more or less the same time as electrophilic attack by molecular bromine. Possible origins of the general base catalysis are discussed. Combined with earlier results, the present findings suggest that a protonated cyclohexadienone is not a mandatory intermediate in phenol bromination; it can be avoided in both the formation of and enolization of the cyclohexadienone intermediate by general catalysis. Keywords: bromination, phenol, mechanism, catalysis, kinetics.